In the electronic industry there is a growing number of new products in display and related areas which are fabricated by high resolution lithography, having patterns whose dimensions are on the order of micrometers, applied over relatively large areas, in the order of 150 to 300 millimeters square. For example, there is an increasing interest in liquid crystal displays (LCDs) for television and computer terminal displays. A typical LCD panel may have 400 by 600 identical pixels, each of 0.5 by 0.5 millimeter in size, covering a total area of 200 by 300 millimeters. Each pixel may have an active element, such as a metal-insulator-metal (MIM) diode device or a thin film transistor (TFT) circuit, the structure of which has critical dimensions in the 1 to 10 micrometers range. The pixels are formed of layers of various materials, such as metals, insulators, and semiconductor materials, which are defined to shapes and sizes corresponding to the elements of the pixel. The layers are defined by a technique which includes coating the layer with a photoresist material and patterning the photoresist using photolithographic techniques. Typically, only a small portion of the total area needs to be exposed at any one time. The exposure system required in the photolithographic fabrication steps must thus be capable of imaging a periodic array of micron size features over a large area with tight tolerances of the feature size and positions. In addition, any practical fabrication process must be capable of high throughput, with exposure times in the order of tens of seconds per panel.
Contact printing techniques, using high resolution, large area masks, can in principle satisfy the above requirements. However, contact printing suffers from the problem of damaged substrate or mask as a result of the intimate contact under pressure required to achieve the high resolution. Non-contact printing requires a lens which is capable of imaging the micrometer dimension features over the large area. Although there are lenses which will have the desired resolution, they are only capable of providing the micron resolution over small areas, about 1 cm.sup.2. Although it is theoretically possible to have a single lens which could provide the desired resolution over the large field area, such lenses would be very large and complex to design and build. Another problem is to achieve accurate focusing of the lens over the large area, particularly if the substrate does not have a completely flat surface.
In the semiconductor industry, where this problem has arisen with regard to making integrated circuits of small feature size on wafers as large as 150 millimeters in diameter, the solution chosen has been to develop wafer-steppers. In wafer-steppers, a high performance lens images a single field at a time of about 1 cm.sup.2 in area, and the total area is exposed by multiple exposures while moving or stepping the wafer across the lens using an accurate x-y translation stage. However, such systems are not capable of high throughput for large area substrates because of the settling and alignment time involved in each step. Printing by means of a focused laser beam scanned across the photoresist through a mechanical or electrical deflection system is subject to the same resolution versus area considerations as lens imaging. High resolution can be achieved by x-y translation of the substrate under high performance focusing optics, but again the translation times involved are not compatible with high throughput systems.